'Could be jailed for war crimes': Reyaad Khan, 20, pictured above in an ISIS recruitment video, has boasted about executions and posted pictures online

A British jihadist could be tried for war crimes and jailed for life after using Twitter to brazenly boast about his role in the ‘execution’ of prisoners in Syria.

Reyaad Khan, 20, has repeatedly uploaded bloodthirsty posts and pictures on social media bragging about the inhumane killings.

In one he posted graphic photographs of the distorted corpses of alleged victims, tweeting: ‘Guys we caught & executed. This is how they looked less than an hr l8er’.

The former college student from Cardiff also laughed about witnessing the ‘longest decapitation ever’ and boasted in another: ‘Executed many prisoners yesterday.’

His claims about his involvement in the crimes carried out by a unit of ISIS are now being investigated by the United Nations.

Khan, who once dreamt of becoming Britain’s first Asian prime minister, could see his name added to the investigators’ list of suspected war criminals in Syria for future prosecution.

Experts believe that Khan, who is one of three young men from Cardiff who joined the group, could also be put on trial for alleged war crimes in a British court if he ever returns to the UK.

Gory: Khan uploaded pictures of men he said his militant comrades had executed, then seemingly disfigured

Propaganda: Kahn (left) appeared in a recruitment video encouraging young men to join ISIS's jihad in Syria and Iraq, alongside Nasser Muthana (centre) and former Aberdeen schoolboy Abdul Raqib Amin (right)

Geoffrey Robertson QC, a former UN appeals judge, said: ‘Any boasts on social media – even a tweet – about killing will be jumped on by the prosecution as a confession and used in evidence.

‘If you fight for Isis, your complicity in war crimes and crimes against humanity would be virtually inevitable.’

He added: ‘Young people contemplating travel to Syria to join a group like ISIS must be warned that if they return they can be prosecuted at the Old Bailey and jailed for long periods for the war crimes and crimes against humanity that these groups are committing.

Share this article

‘Under an 1861 law, guilt of even one murder of a foreigner is enough to land them in jail for life.’

ISIS, which has recently changed its name to Islamic State, announced last month it has formed a caliphate across Syria and Iraq.

Firepower: Boasting tweets posted by Khan include this picture of a modified machine gun

Risk: A QC and former UN judge said that tweets such as the above could lead to prosecution for murder

Up to 500 British Muslims are believed to be in the region with as many as half joining forces with ISIS, a group known for its extreme violence, including crucifying and beheading its enemies.

Khan’s Twitter claims are being examined by the UN after details of his posts were passed on to investigators by the Sunday Times.

The Welsh youngster who left for Syria earlier this year was first exposed as a jihadist in June after he appeared in an ISIS propaganda video brandishing a rifle urging other Britons to join him.

Just last month, he used his Twitter account to boast about his part in the killings of rivals from the Islamist group Jabhat al-Nusra (JN) while fighting in the city of Deir Ezzor in Syria.

Using his alias Abu Dujana RK he wrote on July 5: ‘Executed many prisoners yesterday, (they) sold their deen (religion) for $70 which they didn't even get.’

Terror: Khan's made flippant remarks about his jihad lifestyle, such as the above reference to suicide bombing

In response, another ISIS fighter said: ‘Epic executions bro, we need to step it up like the brothers in Iraq.’

Three days later on July 8, Khan, a former A-grade student, wrote: ‘Probably saw the longest decapitation ever. And we made sure the knife was sharp. Brother who was next decided to the use the glock (pistol) lol.’

When some of Khan’s Twitter followers morbidly asked for photographs, he replied: ‘Don't hav pics. Just vids. Not allowed to post. Bros un-masked in vid having fun.’

On July 16, Khan, whose parents have begged for him to return to their home in Riverside, posted several bloody images of bloated and disfigured corpses.

His tweet added: 'JN guys we caught & executed. This is how they looked less than an hr l8er. Still think JN r knights of the Ummah (Muslim community)?’

Killings: Khan says he saw a man's head slowly removed, then includes 'lol', the acronym for 'laugh out loud'

When one of his 1,500 followers, questioned the authenticity of the photos, Khan replied: ‘I was there when they got killed.’

Days later Khan also posted a picture of an automatic assault rifle with the caption, ‘Check this little badboy out’.

And on July 20 he asked: ‘Anyone want to sponsor my explosive belt? Gucci give me a shout’

The head of the UN’s commission of inquiry on Syria, Paulo Pinheiro, revealed last week that UN investigation teams were currently compiling an exhaustive dossier on war crimes suspects including details of ISIS fighters.

Mr Pinheiro said: ‘I can assure you we are now collecting information on perpetrators from all sides including non-state armed groups and ISIS.’

The UN human rights office added: ‘It is deeply tragic that young men like this are coming from abroad and getting sucked into this terrible conflict where the most appalling atrocities have become commonplace.

‘We will pass on any information we receive of serious crimes to the independent commission of inquiry on Syria.’